Composite materials exhibit high strength and stiffness, as well as corrosion resistant properties. In addition, their light weight is particularly advantageous when compared to similar components constructed from metals. As such, there has been increasing interest in recent years in the use of parts and assemblies constructed from fiber reinforced composite materials in industries such as, for example, the aerospace industry, where parts and assemblies having high strength to weight ratios are desired.
Whether the resin has been infused between reinforcing fibres, or sheets are provided which have been pre-impregnated with resin (commonly and hereinafter referred to as “pre-preg” sheets), the manufacturing of composite structures requires that resins be cured in situ with layered reinforcing fibres. To produce a part or assembly exhibiting the above-described advantageous properties, curing must result in low porosity (i.e., a low number of voids within the composite structure) and a high and substantially uniform degree of cure throughout the entire composite structure.
Curing of resin in a composite structure commonly involves heating the structure so as to induce a cross-linking reaction between molecules of the resin, and a resulting increase in resin viscosity. Ideally, heating is continued until the increase in viscosity of the resin reaches a point whereat gelation occurs, such that the structure has solidified.
Prior art processes used to cure resin in composite structures are not adapted to adequately control the heating of the structure to achieve an optimal uniform level of cure throughout most composite structures, particularly those having more complex cross-sectional shapes. Further, prior art processes yielding products with consistently high quality and strength have required lengthy cure cycles, often in excess of 150 minutes per part. Thus, the inability to provide a cure system capable of quickly and uniformly heating the composite structure so as to achieve a uniform level of cure throughout has been a limiting factor in the use of composite structures in, for example, the aerospace industry.
Prior art curing processes have exhibited the additional disadvantage of being accompanied by high costs, due in large part to the fact that their use necessitates consumption of relatively large quantities of energy.
Moreover, the curing of resin in composite structures having cross-sectional thicknesses greater than about 1.0 inch in any cross-sectional plane (hereinafter referenced in this specification as “thicker cross-sections”) has heretofore been particularly problematic, since a specific and controlled rate of heat is required to cure each such thicker cross-sections to achieve the same degree of cure therein as in other areas of the composite structure at the end of the cure cycle.
It is well known in the art to cure resin in composite structures using an autoclave. Traditionally, curing systems including autoclaves have been the most common means of producing high strength and high quality composite parts. In such processes, a resin impregnated structure is placed in the autoclave, then heated gas at a raised temperature and pressure flows from an inlet end to an outlet end, to thereby heat the composite structure by convective currents circulating within the autoclave. Temperature can vary greatly from one location to another within the autoclave, and no control is typically provided over this variation. For example, the side areas of an autoclave tend to be cooler than the middle areas of an autoclave. As such, the temperature cannot be precisely controlled in all areas of an autoclave and, more importantly, at specific locations throughout the composite structures produced using such systems. This is particularly problematic with respect to composite structures having multiple cross-sectional thicknesses throughout, which ideally require differing rates of heat to be applied at different locations in order to each reach a high and uniform degree of cure. Thus, such controlled differential heating cannot be effectively carried out in prior art autoclave-based systems.
In addition, the use of convective heating means, such as autoclaves, is inefficient in terms of production cycle times, and in terms of energy consumption. This is so for several reasons, including but not limited to the following: i) a long warm-up period is required to bring the autoclave up to its critical operating temperature (at which cross-linking of the resin occurs); ii) a large quantity of energy must be expended to maintain the large volume of the autoclave at temperatures suitable for use in a curing system; iii) a long cure period is required to ensure that the cross-linking is complete throughout all locations of the composite structure; again, for composite structures having thicker cross-sections this is particularly troublesome, and process engineers will typically err on the side of caution in this regard by increasing the cure period; and, iv) a long cool down period is required before the cured composite structures can be safely removed from the autoclave for further production processing. Of course, a long warm up period is again required for the next part or batch of parts to be cured. Thus, in autoclave-based systems, curing times in excess of 150 minutes (exclusive of any necessary cooling time) are relatively common. This, of course, limits the number of composite parts or assemblies that can be produced in any given period of time.
Moreover, given the broad disparity between the volume of the composite structure (and that of any tooling which may be provided thereabouts) and that of the autoclave, the inefficiency of an autoclave from the standpoint of energy consumption per curing cycle is staggering.
It should further be noted that autoclave-based curing systems exhibit yet a further disadvantage, in that they require a very large initial capital investment to build and install. This cost, coupled with high ongoing operating costs, including increasing energy costs, represent a significant barrier to the more widespread use of composite parts and assemblies. Moreover, in an age of perhaps diminishing natural resources, any means of reducing energy consumption is advantageous; quite apart from monetary concerns.
It is desirable, from both quality control and safety standpoints, that all of the resin in the curing of composite structures, whether cured in an autoclave or otherwise, be cured to a substantially uniform level throughout, regardless of the variations in cross-sectional thickness and geometry throughout such structures. Thus, one further significant limitation of prior art curing processes, including those using autoclaves, is the difficulty of consistently achieving the aforesaid uniform level of cure throughout the structure, which is required in order for the final product to have the aforesaid quality and safety. As previously stated, complete and consistent curing of the resin in the structure becomes increasingly difficult as the cross-sectional thickness of the structure varies as between regions of the part. Ideally, thicker cross-sections, and indeed portions having different magnitudes of cross-sectional thickness, require the application of different and controlled rates of heating during the curing process, in order to uniformly cure all portions of the structure to substantially the same degree within a given cure cycle.
Attempts have been made in the prior art to develop curing systems which mitigate the disadvantages of using only an autoclave as their heat source. For example, U.S. Pat. No. 4,828,472 (Itoh et al.), issued May 9, 1989, discloses the use of elemental heaters positioned throughout a mould, which mould is placed in an autoclave environment; however, the elemental heaters of Itoh et al. are merely a supplemental source of heat for curing the workpiece. Thus, the aforementioned disadvantages inherent to autoclave-based curing systems, particularly the high costs (i.e., energy and otherwise) of using same and slow process times, are still experienced with the Itoh et al. system. Moreover, U.S. Pat. No. 4,828,472 does not disclose variable heating and control of the elemental heaters, which variation and control is necessary to achieve high, uniform levels of cure in composite structures having thicker cross-sections, or multiple varying cross-sectional thicknesses.
In addition, the use of elemental heaters such as those discussed in U.S. Pat. No. 4,828,472 (or other conductive heating means) as the primary heat source for a curing system does not substantially mitigate the aforementioned disadvantages of the prior art as related to energy consumption.
Thus, for the reasons mentioned above, amongst others, it has not been practical or economical (for reasons of, among other things, high energy consumption, as discussed above) using known prior art systems or techniques to cure resin in composite structures having thicker cross-sections and/or large thickness variations. There thus continues to exist in the prior art, amongst other things, a need to address these and other limitations, which need is increasing over time as, for example, the aerospace industry looks to increase the variety, complexity and size of composite parts and assemblies used in the construction of airplanes and spacecraft to, amongst other things, reduce weight 62, fuel consumption and cost.
It is thus an object of this invention to obviate or mitigate at least one of the above mentioned disadvantages of the prior art.